Return of VRSA?

A very unsettling report in Lancet today from Portugal, of what is apparently the first appearance of vancomycin-resistant S. aureus in Europe.
Vancomycin is the principal treatment for systemic MRSA infections. A broad-spectrum antibiotic, it is often prescribed on the basis of symptoms and in the absence of any microbiology data. As a result, it is the most-prescribed antibiotic in US hospitals.
Given this history, it is expected that resistance should emerge. About 20% of the population carries S. aureus, and so exposure of Staph strains to vanco is continuous and widespread, even in patients who do not have Staph infections. These are the perfect conditions for the development of resistance. Indeed, the rate of vancomycin resistance in another common Gram-positive pathogen, Enterococcus, has risen to some 30%.
Yet outright vancomycin resistance in S. aureus has remained vanishingly rare. A few cases emerged in Michigan and Pennsylvania in the early 2000's, but were contained and have not re-emerged. In response to this outbreak, the NIH chartered the Network on Antimicrobial Resistance in S. Aureus (NARSA) in an attempt to get out in front of a potential epidemic of untreatable Staph infections.
The epidemic never materialized, and no one is sure exactly why. The gene that confers vancomycin resistance in most Enterococcus strains, vanA, also produces resistance when introduced into S. aureus, and was found in the VRSA isolate described in the Lancet paper. S. aureus strains that harbor vanA are not obviously less fit. Even if they were, they would be expected to evolve - the first MRSA strains grew very poorly and were quickly outcompeted by susceptible strains in the absence of methicillin selection. S. aureus does not readily take up DNA from other organisms in its environment. But Staph and Enterococcus are frequently found together in wounds (as they were in the Portugese case) and other infections, and even once in a trillion events like vanA gene transfer must have occurred many millions of times.
We've been virtuous (chartering NARSA was an act of prudence and foresight), but mostly lucky. An outbreak of VRSA that spread like MRSA would create an epidemic of truly terrifying proportions. Almost everyone expects it to happen eventually. The question is whether we will use the respite we've been granted to develop effective alternatives to vancomycin when it begins to fail.

Posted with Blogsy